Potential impact of a low-level SAAF Oryx helicopter ... · Potential impact of a low-level SAAF Oryx helicopter flight on Cape fur seal pup mortality in the Robberg Marine Protected

THE POTENTIAL IMPACT OF A LOW-LEVEL HELICOPTER FLIGHT VS.

ENVIRONMENTAL FACTORS ON CAPE FUR SEAL MASS PUP MORTALITY IN THE

ROBBERG MARINE PROTECTED AREA, PLETTENBERG BAY

An investigational report for CapeNature, Bitou Municipality and the South African Air Force

by

JSF van der Vyver and DS Conry

For queries email [email protected] or visit www.wildocean.org.za

Submitted February 2020

mailto:[email protected]

Potential impact of a low-level SAAF Oryx helicopter flight on Cape fur seal

pup mortality in the Robberg Marine Protected Area 2020

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SUMMARY

The potential impact of an anomalous disturbance event caused by a low-level SAAF Oryx

helicopter flight over the Cape fur seal breeding colony on Robberg Peninsula was examined using

seal stranding records and scientific observation. Varying states of decay in carcasses washed

ashore over the November 2019 – January 2020 breeding season was used to estimate the likely

date of death in 553 newborn pups. Using this data an estimated (albeit crudely) 47 newborn pups

may have landed up in the water and drowned due to disturbance caused by the helicopter flight on

30 November 2019, although there is no causal proof. Available data and preliminary observations

suggests the other 506 newborn pups most likely drowned as a result of disturbance caused by

territorial bull fights, pups falling off steep cliffs and the onslaught of summer storms that knock

pups off the rocks – especially during periods of high easterly wind and swell. Even though summer

storms have been identified as the main factor causing high levels in pup mortality at other

breeding colonies along the south coast of South Africa, further statistical analysis into the effect of

various environmental factors are needed to fully test this hypothesis at the Robberg breeding

colony. Emerging evidence of an increasing trend in pup production at Robberg is most likely

related to a local increase in the availability of preferred prey (i.e. small pelagic fish), leading to an

increase in body condition and therefore reproductive rates of females. A possible influx of adult

females from other colonies may also be a contribution factor and could be investigated in future

using satellite tracking devices, if funding becomes available. Any further increases in the observed

pupping trend will likely be mirrored by an increase in easterly storm-related mortality, and

therefore carcasses washed ashore.

This report may be cited as:

van der Vyver JSF and Conry DS. 2020. The potential impact of a low-level helicopter flight vs. environmental factors

on Cape fur seal mass pup mortality in the Robberg Marine Protected Area, Plettenberg Bay. Investigational report

prepared for Cape Nature, Bitou Municipality and the South African Air Force. 25pp. Available online at:

http://www.wildocean.org.za.



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TABLE OF CONTENTS

ABBREVIATIONS.............................................................................................................................4

FIGURES............................................................................................................................................5

1. INTRODUCTION.......................................................................................................................6

Geographical distribution........................................................................................................6

History......................................................................................................................................7

Breeding season.......................................................................................................................8

2. AIMS AND OBJECTIVES.......................................................................................................10

3. MATERIALS AND METHODS..............................................................................................10

Study site................................................................................................................................10

Helicopter disturbance...........................................................................................................10

Recovery of carcasses.............................................................................................................11

Environmental data................................................................................................................11

Live pup counts......................................................................................................................12

Descriptive statistics...............................................................................................................12

4. PRELIMINARY RESULTS.....................................................................................................12

Helicopter disturbance...........................................................................................................12

Stranding dates.......................................................................................................................13

Carcass categorization...........................................................................................................14

Potential date of death............................................................................................................15

Comparison of spikes in pup strandings over previous seasons............................................15

Impact on live pup counts.......................................................................................................16

5. DISCUSSION.............................................................................................................................17

6. CONCLUSION..........................................................................................................................19

7. RECOMMENDATIONS..........................................................................................................20

Municipal carcass disposal....................................................................................................20

Future research and monitoring............................................................................................20

ACKNOWLEDGEMENTS.............................................................................................................21

REFERENCES.................................................................................................................................21



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ABBREVIATONS

CN – CapeNature

DEFF – Department of Environment, Forestry and Fisheries

MPA – Marine Protected Area

PEM – Port Elizabeth Museum

RNRC – Robberg Nature Reserve Complex

SAAF – South African Air Force



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FIGURES

Figure 1: The range of Cape fur seal breeding colonies from Algoa Bay in South Africa to Baia

dos Tigres in southern Angola.

Figure 2: A historical representation of the number of Cape fur seals hauled out on Robberg

Peninsula between 1833 and 2020.

Figure 3: Satellite image of Robberg Peninsula showing the extent of the seal colony at the

beginning of the recolonisation period, after 2008 and after 2010. The location of

pupping sites observed during dedicated boat-based counts between 2017 and 2020

are included.

Figure 4: a) Stranding dates of 620 Cape fur seal carcasses that washed ashore on Plettenberg

Bay beaches during the 2019/2020 breeding season, including all age classes from

pups to yearlings, subadults and adults, and b) daily wind and swell conditions over

this period.

Figure 5: The external appearance of a tagged Cape fur seal pup (GG0378) that washed ashore

a second time on Robberg beach on 12 December 2019, after drowning at the

breeding colony on 7 Dec 2019 and being submerged for ~6 days.

Figure 6: a) Estimated date of death of 620 Cape fur seal carcasses that washed ashore on

Plettenberg Bay beaches during the 2019/2020 breeding season, and b) a range of

daily environmental conditions over this period.

Figure 7: Spikes in newborn Cape fur seal pup carcasses washed ashore over three breeding

seasons between 2017 and 2020.

Figure 8: Peaks in live Cape fur seal pup counts over three breeding seasons between 2017 and

2020.

Figure 9: Emerging trends in Cape fur seal minimum pup production estimates based on a

combination of maximum live counts at the colony and total carcasses washed ashore

at the time of those counts.



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INTRODUCTION

Geographical distribution

The Cape fur seal Arctocephalus pusillus pusillus is the only resident pinniped species along the

southern African coastline. The current geographical distribution of its breeding population ranges

from Algoa Bay in South Africa to Baia dos Tigres in southern Angola (Kirkman et al. 2011). The

latest total population estimate suggests some 2 million animals, ~99% of which occurs along the

west coast of southern Africa. According to the definition of a Cape fur seal breeding colony as a

location where at least 100 pups per year are born regularly (Oosthuizen and David 1988), there are

currently 41 breeding colonies in the population (Kirkman et al. 2011, PEM unpublished data). The

majority of these are associated with the Benguela Current system located along the west coast of

the region, with only three breeding colonies occurring to the east of Cape Agulhas, in the Agulhas

Current system (Figure 1). These include Black Rocks in Algoa Bay, Robberg Peninsula in

Plettenberg Bay and Seal Island in Mossel Bay.

Fig. 1. Map showing the range of Cape fur seal breeding colonies from Algoa Bay in South Africa

to Baia dos Tigres in southern Angola. Breeding colonies along the south coast include Black

Rocks in Algoa Bay (BR), Robberg Peninsula in Plettenberg Bay and Seal Island in Mossel Bay

(SIMB). Selected breeding colonies along the west coast include Geyser Rock (GR), Seal Island

False Bay (SIFB), Vondeling Island (VI), Kleinsee (KS), Atlas Bay (AB), Hollam’s Bird Island

(HBI), Cape Cross (CC), Cape Frio (CF) and Baia dos Tigres (BT). (taken from Huisamen et al.

2011.)



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History

The current geographical distribution of the south coast Cape fur seal population differs

considerably from its historical distribution, as reconstructed from records or anecdotes of seal

hunters and early travellers (e.g. Rand 1972, Shaughnessy 1982, 1984). Historically, at least nine

seal colonies occurred to the east of Cape Agulhas, but most of these were hunted to extinction prior

to the 20th century, including at least five colonies in Algoa Bay and another two in Plettenberg

Bay (Shaughnessy 1982, Stewardson 1999a). In Plettenberg Bay specifically, Cape fur seals

historically hauled out on Seal Point at Robberg and also Beacon Island (long before being

developed and joined to the mainland) in large numbers as far back as the 15th century. This is

evident in many ship records from early travellers and seal hunters that visited the bay. In fact,

remains from Khoi middens investigated by archeologists in Nelson Bay cave suggest that Cape fur

seals have been present at Robberg Peninsula for over 8000 years (Sealy 2006). In August 1833, an

anecdotal record from an observation made by a government official suggests that there were at

least 3000 seals hauled out on Robberg Peninsula (Ross 1971). At the time Cape fur seals were

already indiscriminately hunted for their fur and blubber, both in Plettenberg Bay and elsewhere

along the east and west coast of South Africa. By 1890, due to uncontrolled harvesting, Cape fur

seals were completely exterminated from both their historical haul-out sites in Plettenberg Bay

(Ross 1971).

Following the introduction of protective legislation in 1893 (Best 1973), and the proclamation of

the Robberg Nature Reserve Complex (RNRC) in 1980, it was only in the early 1990’s that small

numbers of seals returned to Robberg Peninsula (Stewardson and Brett 2000). Numbers

subsequently increased (Stewardson 2001) and a few newborn pups were first observed in

1996/1997, leading to speculation that Robberg could eventually become a breeding colony

(Stewardson 1999, Kirkman 2010). Due to the potential for future impacts on local fisheries

(Wickens et al. 1992) and the conservation of certain other marine top predators (Kirkman 2009), as

well as the colony’s ecotourism potential, it was identified as a monitoring priority by the Robberg

management authority (CapeNature). Assisted by CapeNature and the Center for Dolphin Studies,

Huisamen et al. (2011) used routine land-based counts to describe the recolonisation process

between 2000 – 2008, including between- and within-year temporal patterns in haulout numbers

and the extent of breeding. Unpublished data from routine boat-based counts performed by

CapeNature since 2009 and independent researchers from Wild Ocean since 2018 (with the support

from the ORCA Foundation and Ocean Blue Adventures) suggests that seal haul-outs at the



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Robberg colony have been steadily increasing, despite large seasonal and annual fluctuations in

numbers (Figure 2). However, with an overall average of ~3500 seals ashore, and similar numbers

recorded during spring, it appears the Robberg colony have once again reached those historical

numbers recorded by a government official in August 1833.

Fig. 2. A historical representation of the number of Cape fur seals hauled out on Robberg Peninsula,

between 1833 and 2020.

Breeding season

The annual Cape fur seal pupping season extends from mid-November to early January with a peak

in births around mid-December. Pups are born in a rookery on land and depend on their mothers for

10-11 months. During this time mothers regularly leave their pups to forage for 2-10 days at a time,

re-uniting for a few days between trips to suckle (David & Rand 1986).

Newborn pups are notoriously susceptible to high natural mortality levels (David 1987, Roux and

de Villiers 1991, de Villiers and Roux 1992, Wickens & York 1997, Kirkman et al. 2006, Kaplan et

al. 2008). In addition, the pupping season for south coast populations coincide with summer storms

that are accompanied by high swells (>3m) (Stewardson 1999a). Whereas such conditions are not a

problem for older seals, newborn pups are not able to swim very well (Rand 1967, David 1995) and

can be swept away from the colony if they are washed off the rocks in rough weather. Many drown

and wash up on the mainland, but live ones are sometimes rescued and returned to their colony

(Stewardson 1999a, 1999b, Hofmeyr et al. 2011). Although such events have been recorded in



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Algoa Bay since 1912 (Stewardson 1999b), it has not been studied in detail at other colonies further

west.

Human disturbance can also have a major impact on the survival of young marine colonial breeding

species since they may be crushed or otherwise injured by fleeing adults, lose contact with their

mothers, be prevented to form a mother-neonate bond (leading to abandonment and starvation), or

fall into the sea and drown. Cape fur seals are no exception, being instinctively afraid of human

activity as they have evolved without human predators. At Robberg Peninsula human disturbances

over the years is thought to have resulted in changes in the distribution of their breeding locations,

leading to the present haulout distribution and pupping sites shown in Figure 3 below.

Fig. 3. Google Earth image (http://earth.google.com/) of Robberg Peninsula showing the extent of

the seal colony along the coast at the beginning of the recolonisation period (between A and B),

after 2008 (between C and D) and after 2010 (between E and D). Red sections indicate the location

of pupping sites observed during dedicated boat-based counts between 2017 and 2020. Blue

sections indicate popular shore angling locations. The most suitable access points for seals on the

north face of the peninsula are marked with arrows. Hiking trails cover vast majority of the coastal



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section along the south coast and is thought to prevent seals from hauling out in numbers along this

section. (adapted from Huisamen et al. 2011.)

AIMS AND OBJECTIVES

This report is presented in support of the investigation into a low-level South African Air Force

(SAAF) Oryx helicopter flight over the Robberg Nature Reserve Complex (RNRC) and the local

Cape fur seal breeding colony on 30 November 2019. Using seal stranding records and scientific

observation the report aims to provide 1) a descriptive overview of the potential impact of the

disturbance event on newborn pup mortality levels in the 2019/2020 breeding season, and 2) a

number of hypotheses explaining the cause of the apparent spike in mortality that was detected in

early December 2019.

MATERIALS AND METHODS

Study site

Robberg Peninsula is part of the Robberg Nature Reserve Complex and is adjoined by a marine

protected area (MPA) which was established during the late 1990s (Government Notice No. 1 of

1980, Schutte-Vlok et al. 2018). The peninsula forms the south-western extremity of Plettenberg

Bay, situated on the southeast coast of South Africa (Figure 1). There, the fast-flowing Agulhas

Current intermittently causes inshore counter currents and upwelling of colder water, especially

after periods of high easterly wind (Schumann et al. 1982, Lutjeharms et al. 2000, Lutjeharms and

Ansorge 2001, Goschen et al. 2012, 2015). However, warm water intrusion events are not

uncommon (Goschen et al. 2012, 2015). Rough sea conditions are generally associated with

westerly cold fronts in winter and strong easterly winds in summer.

Helicopter disturbance

An anomalous disturbance event that was recorded via direct observation and a video recording

provided the only factual evidence of the event. The observer was interviewed and the video

material studied to collect information of his account.



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Recovery of carcasses

As part of a broader Cape fur seal monitoring programme operating under research permits issued

to the Port Elizabeth Museum (PEM) by the Department of Environment, Forestry and Fisheries

(DEFF), and CapeNature (the Robberg management authority), data collected in Plettenberg Bay

since 2017 allow the assessment of 1) changes in the frequency of breeding season mortality levels

through the recovery of information from carcasses washed ashore, and 2) minimum pup production

estimates through a combination of dedicated boat-based counts and stranding records. In order to

obtain comparable values we have taken each breeding season as starting on 1 November and

ending on 31 January. During this period daily beach surveys and/or spot checks were performed on

Robberg, Central, and Lookout beaches. The more isolated Keurbooms beach was only surveyed 3-

4 times per week. As a result, a small portion of pup carcasses washed up on this beach may have

spent a day or two ashore before being discovered. Between 2017 and 2018 pup carcasses were

buried in order to prevent re-sampling. Since 2019 they were marked with either brightly coloured

spray-paint or plastic flipper tags. Carcasses were usually removed by municipal authorities less

than one week after washing ashore, but in some cases up to two weeks later. Carcasses found

washed up on more remote beaches such as Keurbooms were also marked with spray-paint or

plastic flipper tags, but due to the low number of beach visitors were left to decay naturally.

Live pups that washed ashore on the mainland were collected by various local authorities and

members of the public, some of which were taken to the Tenikwa Wildlife Rehabilitation and

Awareness Centre for short term treatment of injuries. Those that survived were double-tagged with

uniquely identifiable plastic roto-tags on the trailing edges of their fore flippers, and with help from

Offshore Adventures returned to the colony by boat in calm weather, usually within 1-7 days of

stranding. Any that washed ashore a second time (dead) can be used to assess the rate of decay and

therefore estimate the possible date of death. The re-stranding of the carcass of one such live pup

that was previously tagged and released at the colony was used to categorize the rate of decay in all

carcasses that washed ashore over the 2019/2020 breeding season.

Environmental data

Wind/gust speed and direction, wave height and direction, air temperature and cloud cover

conditions over the study period were extracted from archived data available on

https://www.windguru.cz/archive.php.

https://www.windguru.cz/archive.php



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Live pup counts

22 separate dedicated boat-based counts of live newborn pups ashore at the colony were made over

three consecutive breeding seasons between 2017 and 2020. Each count was carried out by an

experienced observer and black pups (defined as newborn animals less than 2 months old) were

distinguished from older animals based on their small size and the morphometric and colour

descriptions of Rand (1956). A standard sheep counter was used to keep tally of all black pups

visible to the naked eye (therefore an undercount of the true number of newborn pups ashore as the

angle of the boat most likely concealed many hiding among the rocks and boulders) while the vessel

(~9m double-hull) slowly travelled along the peninsula at a distance of 10 – 20m from the high-

water mark. Surveys started at site E near the western edge of the peninsula and ended at site D near

the eastern edge of the peninsula (Figure 3). On average counts lasted 40 minutes and were carried

out between 09:00 and 15:00. A range of environmental factors such as sea surface temperature,

wind direction, beaufort scale and cloud cover conditions were also recorded for inclusion in future

analyses on their potential effects.

Minimum pup production estimates were calculated by combining the highest count of the season

(although underestimate) with the total number of pup carcasses recorded ashore along Plettenberg

Bay beaches up until the date of that count.

Descriptive statistics

No advanced statistical analyses was applied as the aim of this report was merely to present a

summary of the data using basic figures in Excel. Data will however be analysed in future using

appropriate software, as part of broader research publications on Cape fur seal biology in the

Agulhas Current.

PRELIMINARY RESULTS

Helicopter disturbance

On Saturday 30 November 2019, at 10:00am, a CapeNature Field Ranger made a direct observation

and video recording of a low-level SAAF Oryx helicopter flight from an elevated viewpoint directly

above the ‘Meidebank’ angling site. Initially the helicopter was heard, and later seen approaching

from Beacon Island in the north heading directly towards the gap area located on the northern side

of Robberg Peninsula. After inspecting the video footage it was clear that once the helicopter



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reached the peninsula it turned east and proceeded to fly in a path parallel to the peninsula, roughly

30-50m from the coastline and below 100m above the surface of the water towards Seal Point

(Robberg Point). The footage shows the helicopter passing by two of the three known seal pupping

sites, but due to the far distance away (~2km) it was not possible to see clearly whether any

stampeding occurred that could have knocked seal pups into the water. It is worth noting that sea

conditions (i.e. wind and swell) were extremely calm on the northern side of the peninsula at the

time of the observation and for the remainder of the day.

Stranding dates

Figure 4 illustrates the stranding dates of all Cape fur seal carcasses washed ashore on Plettenberg

Bay beaches during the 2019/2020 breeding season, including daily wind and swell conditions.

Although stranding dates of older age classes are more evenly distributed throughout the breeding

season, with an increase during January, a clear peak in pup strandings is apparent roughly 9-10

days after the helicopter disturbance event took place on 30 November 2019.

Fig. 4. Illustrates a) the stranding dates of 620 Cape fur seal carcasses washed ashore on Plettenberg

Bay beaches during the 2019/2020 breeding season, including all age classes from pups to

yearlings, subadults and adults, and b) daily wind and swell conditions over this period.



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Carcass categorization

An unfortunate event involving the rescue, tagging and unsuccessful release of a live stranded

newborn pup at the breeding colony on Robberg Peninsula led to its drowning on 7 December 2019.

The carcass was found washed ashore on 12 December 2019, allowing photographic evidence of

the external appearance of a drowned newborn pup that had been submerged in seawater for ~6

days (Figure 5). As the condition of >90% of stranded carcasses were assessed on the day they

washed up, this information was used to distinguish between varying states of decay in 553

newborn pups that washed ashore over the 2019/2020 breeding season, and to estimate their likely

date of death (albeit crudely). Carcasses with very little to no fur and a similar appearance to that in

Figure 5 was scored 7 days dead, those with 2/3rds

of their fur missing 5 days dead and those with

~1/3rd

of their fur missing 3 days dead. Pups with all their fur still attached were scored with either

being one or two days dead depending on other external features associated with their eyes and

flipper tips.

Fig. 5. The external appearance of a tagged Cape fur seal pup (GG0378) that washed ashore a

second time on Robberg beach on 12 December 2019, after drowning at the breeding colony on 7

Dec 2019 and being submerged for ~6 days.



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Potential date of death

After stranding dates were corrected for estimated date of death, taking into account the state of

decay of each pup that washed ashore, the preliminary assessment (albeit crudely) suggests 47 pups

drowned on the day of the helicopter disturbance event (Figure 6).

Fig. 6. a) Estimated date of death of 620 Cape fur seal carcasses that washed ashore on Plettenberg

Bay beaches during the 2019/2020 breeding season, and b) a range of daily environmental

conditions over this period.

Comparison of spikes in pup strandings over previous seasons

Figure 7. illustrates a comparison of spikes in pup carcasses washed ashore over three breeding

seasons between 2017 and 2020. Although spikes are comparable, a major increase in strandings in

the 2018/2019 and again 2019/2020 breeding season is apparent.



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Fig. 7. Spikes in newborn Cape fur seal pup carcasses washed ashore over three breeding seasons

between 2017 and 2020.

Impact on live pup counts

Figure 8. illustrates a dates and numbers of 22 live pup counts performed over the past three

breeding seasons (2017 – 2020), showing a clear increase in maximum counts during the two most

recent breeding seasons (i.e. 2018/2019 and 2019/2020).

Fig. 8. Peaks in live Cape fur seal pup counts over three breeding seasons between 2017 and 2020.

Figure 9 shows an increasing trend in Cape fur seal pup production and newborn pup mortality over

the past three breeding seasons. The blue circle highlights the unexpected drop in live pup counts

and a higher than expected increase in dead pups washed ashore over the 2019/2020 breeding

season.



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Fig. 9. Emerging trends in Cape fur seal minimum pup production estimates based on a

combination of maximum live counts at the colony and total carcasses washed ashore at the time of

those counts.

DISCUSSION

Figure 4 shows a clear mismatch between the helicopter flight disturbance event on 30 November

and the spike in strandings of newborn pup carcasses between 9 and 10 December. Although the

mismatch could be due to the delay from disturbance to actual drowning of pups and subsequent

strandings on Robberg beach, the delay may also be influenced by the timing and strength of

easterly winds and swells which are believed to be the main factors responsible for casting

carcasses ashore on Plettenberg Bay beaches. In Figure 4 it is clear that the peak pup stranding

period over 9-10 December coincides with an extended period of storm conditions characterised by

strong easterly wind coupled with high southerly, south-easterly and especially easterly swells.

Therefore, the spike in estimated newborn pup mortality presented in Figure 6 may be attributable

to one or more of the following: (1) the helicopter disturbance had resulted in a stampede of

panicking adults that caused a large number of newborn pups to get knocked off the rocks and land

up in the water where they drowned and washed ashore concurrently, (2) pups had been washed off

Robberg Peninsula due to a major easterly storm, drowned and washed ashore concurrently, (3)

pups had drowned over a number of days previously, due to a variety of environmental conditions,

but due to a combination of easterly wind and wave conditions were washed ashore concurrently.



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The helicopter disturbance event can be ruled out as estimates of the date of death in pups that

washed ashore on 9 and 10 December suggests that the vast majority drowned between 2-6

December, some 2-6 days after the helicopter disturbance event.

It is important to remember that high rates of mortality prior to weaning are not unusual in seal pups

(Wickens & York 1997, Kaplan et al. 2008). Furthermore, catastrophic events, such as

displacement and drowning due to storms have been recorded in a number of species, including

grey seals Halichoerus grypus (Le Boeuf and Briggs 1977, Anderson et al. 1979, Barnett and

Westcott 2001), southern elephant seals Mirounga leonine (Carrick and Ingham 1962), northern

elephant seals M. angustirostris (Reiter et al. 1981, Riedman and Le Boeuf 1982), harbour seals

Phoca vitulina (Boness et al. 1992), Steller sea lions Eumetopias (Kaplan et al. 2008, Maniscalco et

al. 2008) and Subantarctic fur seals Arctocephalus tropicalis (Beauplet et al. 2005). Cape fur seal

pups are also subject to high mortality rates in the first few months of their life (David 1987, Roux

and de Villiers 1991, de Villiers and Roux 1992), including drowning of pups that are washed from

rookeries by large swells (Stewardson 1999a, Kirkman et al. 2006). Storm-related mortality is also

of importance for the conspecific Australian fur seal A. p.doriferus (Pemberton and Kirkwood 1994,

Arnould and Littnan 2000, Hume et al. 2001). Mortality of seal pups due to catastrophic

meteorological events varies with the topographic characteristics of the haulout area (Le Boeuf and

Briggs 1977, Riedman and Le Boeuf 1982, Doidge et al. 1984, Boveng et al. 1998, Baker et al.

2006); some rookeries experience very little mortality during storms because the topography of the

haulout area provides adequate protection (de Villiers and Roux 1992, Pemberton and Kirkwood

1994, Boveng et al. 1998, Gucu et al. 2004). By contrast, the pupping locations of Cape fur seals on

the northern side of Robberg Peninsula offers very little protection during easterly storms. The

location is also blocked off from cool westerly winds causing extreme air temperature conditions

during such periods in summer. Newborn pup mortality during such events is therefore expected to

be high.

Although peaks in high easterly wind/gust and swell, and high air temperatures during the

2019/2020 breeding season appears to correlate with spikes in estimated pup mortality (Figure 6),

the true effect of these factors will only be examined in future analyses (using appropriate software)

in order to fully test the other two hypotheses.



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The increasing trend in newborn pup mortality is consistent with emerging trends in minimum

estimated pup production and recent growth of the colony (CapeNature and PEM unpublished data).

Results on the potential impact of the helicopter flight are also consistent with the unexpected

decrease in 2019/2020 peak live pup counts and a large increase in the total number of pup

carcasses washed ashore, which was most likely influenced by this anomalous disturbance event.

CONCLUSION

Preliminary results presented here suggest disturbance caused by the low-level SAAF Oryx

helicopter flight on 30 November 2019 had a minor impact on overall pup mortality during the

2019/2020 Cape fur seal breeding season, potentially causing the death of up to 47 newborn pups.

Although sea conditions were calm on the day of the incident, any newborn pups that may have

landed up in the water near the steep sections of the peninsula would have had very few suitable

sites to haul out onto the safety of the rocks, and many would have inevitably drowned in their

attempts.

Based on the data it appears the peak pup stranding period of 9-10 December 2019 was most likely

related to newborn pups that had drowned over a number of days previously, mainly between 2-7

December with a peak in mortality on 4 December, and due to a combination of strong easterly

wind and swells were washed ashore concurrently. Future data analysis will investigate the effect of

various environmental factors in order to test this hypothesis.

It is important to acknowledge emerging evidence of an increase in pup production at this breeding

colony, most likely related to a local increase in the availability of the Cape fur seal’s preferred prey

species (i.e. small pelagic fish such as anchovy, sardine and horse mackerel after Huisamen et al.

2012). Previous studies have shown how the quantity and quality of prey effects the body condition

of marine top predators, including seals, which effects the timing of puberty, implantation of

embryos, mortality of foetuses, and therefore reproductive rates of females (reviewed in Hazen et

al. 2019). Based on this it is very possible that the increasing trend in Cape fur seal pups washed

ashore on Robberg beach over the past three breeding seasons (2017 - 2020) may be related to an

increase in the reproductive rates of females. However, the potential influx of adult females from

other areas may also be a contributing factor. Either way, it is suspected that any further increases in



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pup production will naturally be mirrored by increases in easterly storm-related pup mortality, and

therefore carcasses washed ashore.

Although not related to the aims and objectives of this report, it is worth mentioning that the cause

of mortality in majority (>90%) of the older age classes (yearlings, subadults and adults; overall

76% male) since 2017 are unknown as most wash up in an advanced state of decay. However, there

are necropsy records for some fresh carcasses which report on various usual and unusual causes of

mortality and secondary symptoms. These include shark inflicted wounds, stingray inflicted

wounds, parasitic infestations, entanglement in fishing gear and plastics, and shot seals. For those

that were fresh enough to collect pathology, the samples are stored with Dr. Greg Hofmeyr at the

PEM until future investigation on toxicology and the presence of disease and virus. The results of

these investigations will be reported on in future publications.

RECOMMENDATIONS

Municipal carcass disposal

As vast majority of seal carcasses were cast ashore during periods of high easterly wind and swell,

it is advised to delay major municipal clean-up efforts until such conditions have passed.

Future research and monitoring

Continued boat-based counts of live newborn pups at the colony and land-based counts of the

number of carcasses washed ashore during the pupping season may prove crucial in studying the

importance of seal birth rate as an indicator of ecosystem health and/or changes lower down in the

food-web, especially if linked to results from future diet analysis of adult seals.

Given the potential of breeding animals moving into the region from other areas, the movement and

exchange of seals between south coast colonies should be investigated using satellite tracking

devices. Numerous funding applications have been submitted for this component over the years, but

to date none have been successful. In the meantime, relatively inexpensive flipper tagging efforts

could be intensified at all breeding colonies along the south coast. Ongoing monitoring of carcasses

washed ashore, and seals drowned in mid-water trawl nets further offshore (Reed et al. 2017, van

der Vyver personal observation) could provide valuable ‘recapture’ data.



21

ACKNOWLEDGEMENTS

This preliminary assessment was conducted as part of a broader research programme on the biology

of Cape fur seals and their interactions with fisheries in the Agulhas Current; and in terms of

permits issued to the PEM by the DEFF and CapeNature. We would like to thank Bitou

Municipality, Offshore Adventures, Tenikwa Wildlife Rehabilitation and Awareness Centre, the

Plettenberg Bay Marine Animal Stranding Network and Ocean Blue Adventures for their logistical

support. Fieldwork and equipment was partially funded by the PEM, the ORCA Foundation and

small donations from members of the public. However, a large portion of the costs were covered by

the authors. No financial compensation was received for preliminary data analysis or report writing.

We thank our research collaborator Dr. Greg Hofmeyr for his insightful comments on an earlier

version of this report.

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Potential impact of a low-level SAAF Oryx helicopter ... · Potential impact of a low-level SAAF Oryx helicopter flight on Cape fur seal pup mortality in the Robberg Marine Protected